This invention relates to the measurement of the surface profiles of components. More particularly, it relates to the measurement of aerofoil blades of a gas turbine engine.
In a gas turbine engine, each of the rotor blades and stator vanes is of aerofoil cross-section shape and must conform with its intended design, within certain limits of acceptability. For example, each aerofoil has a leading edge designed with a complex geometry to deliver a specific effect. Deviations from the optimum design may result in undesirable reductions in performance, such as aerodynamic stall, compressor surge or reduction in engine efficiency. In some circumstances, relatively small deviations may have significant effects on performance.
The assessment of conformity of aerofoil edges is conventionally done by comparing a set of measured data points to the nominal design profile requirements. The design profile has a tolerance band within which the data points must lie in order for the blade to be deemed acceptable. The tolerance band can take a number of forms—unilateral, bilateral, constant width, variable width, etc. The assessment of form is independent of the assessment of edge position, so it is normal practice to do some form of best fitting of the measured data to the nominal data before making the assessment. The standard method of best fitting is the least-squares error method, and following the best fit all the points of the measured contour must lie within the tolerance band in order for the blade to be acceptable. European patent application EP1615153A2 describes one such assessment method.
This method of assessment does not take full account of the relative importance of the region of the aerofoil near to the edge in comparison to the regions further away. This is particularly important when the edge profile is not designed to be circular, for example when it is elliptical. The least-squares method is also limited when the blade is measured part way through manufacture, and has material which is yet to be removed, or where the blade shape will change due to the manufacturing process. Additionally, comparison of a full-form profile with the tolerance bands is time-consuming.
Conformity may alternatively be assessed using mechanical measuring devices such as go/no-go gauges, but these can damage the surfaces they are designed to measure.